The invention relates to soil plugs for use in sprouting seedlings for later transplantation. In particular, it relates to a method and apparatus for forming elastomeric soil plugs from a slurry of soil and polymer resin.
Soil plugs or other compact soil masses are widely used in crop transplanting systems. In such systems seedlings are grown in high concentration in a greenhouse under controlled conditions which assure a high starting rate. The seedlings, started from seeds planted in the soil plugs or masses, are then transplanted to the field when the seedlings reach a certain level of maturity. In the transplant approach to crop cultivation, the transplants can be spaced apart in the field a predetermined distance to provide sufficient, but not excessive, growing room per plant, thereby giving maximum yields per acre of valuable farmland. In the traditional and more wasteful direct seeding techniques, by contrast, one first overplants a field with seeds (not seedlings) and then thins to the optimal growing density the young plants managing to sprout.
Although the transplanting method has proved to be an advance over the direct seeding technique, it is not free from problems. Seedlings have delicate root systems which are especially vulnerable to handling and mechanical shock. This vulnerability makes it especially difficult to transplant the seedlings by automated techniques without the loss of some portion of them. The lost seedlings must then be replaced--usually by hand--if the farmland is to yield a full crop.
The recent development of elastomeric soil plug technology has greatly facilitated the automated handling and transplanting of seedlings. Elastomeric soil plugs are plugs formed from a mixture of soil and an elastomeric, biodegradable, polymer resin, which binds the soil together and provides it with some resiliency. When a seed planted in such a plug sprouts, its root system permeates the elastomeric soil composition, which has sufficient resiliency to enable machine handling of the seedling root system without killing the plant.
Elastomeric soil plugs and several ways of manufacturing them are disclosed by Dedolph in his pioneering U.S. Pat. Nos. 4,034,508 and 4,130,072. In one method of manufacture, two sets of semi-cylindrical mold elements are carried along endless conveyers. During a portion of the path of travel of the conveyers, corresponding sets of mold elements join to form a set of cylindrical mold cavities for the soil plugs. The mold cavities are automatically filled with a slurry of organic pre-polymer compound and soil mix. The slurry cures as the mold cavities are carried along the conveyers, and after a sufficient run of the conveyers the mold elements separate to release the fully cured soil plugs. To facilitate handling of the plugs, Dedolph teaches the method of interconnecting them by inserting a narrow strip of flexible material between the two sets of mold elements as they come together. In this way an interconnected string of plugs is formed. The plugs are seeded and the seeds germinate, while the plugs are interconnected. When the seedlings are ready for transplanting, the interconnecting strip is severed and the seedlings planted in the ground.
According to another method of manufacture disclosed by Dedolph, a top mold member defining a plurality of mold cavities is positioned on top of a bottom mold member. A cam engaging the bottom mold member from below regulates the relative position of the two mold members and consequently the depth of the mold cavities. The soil-resin slurry is poured into the mold cavities for curing. The soil-resin mix is such that as it sets up, it expands. Dedolph accommodates this expansion with the regulating cam; as the mixture expands, the cam allows the top and bottom mold members to separate. After the plugs have fully cured, the two mold members can be squeezed together to eject the finished plugs. This method of manufacture can also be used to form two-dimensional arrays of plugs interconnected by flexible strips.
For efficient soil plug operations--whether they be seeding, transporting or transplanting--the plugs must be handled in large blocks or units rather than individually. A unit formed by interconnecting plugs with a flexible strip or strips, as taught by Dedolph, poses handling problems. The root systems of the developing seedlings tend to get bound to or entangled with the flexible strips and, consequently, are likely to be damaged when the strips are severed.
Another method of forming soil plugs in large blocks starts with a tray having a plurality of cavities in it, which serve as molds for individual plugs. After the plugs have been formed within the cavities, the tray serves as a matrix to hold the plugs during later operations. The plugs are seeded within the trays, the seedlings develop within the trays, the trays are moved from the greenhouses to the field, and they are loaded onto machines which automatically remove individual seedling plugs and plant them in the ground.
In the past, soil plugs have been formed within such carrying trays in the following manner. The carrying tray itself forms one part of a mold, referred to herein as the tray mold member. Cooperating with the tray mold member is a base mold member, which has an array of projections on one surface. When the base and tray mold members are indexed to one another, the array of projections protrude into a corresponding array of plug molding cavities within the tray mold member. These cavities are generally frustoconical in shape; with the mold members in their indexed configuration, the base member seals the broader ends of the cavities and its array of projections extend therein a measured distance to define seed cavities in the molded plugs.
To mold the soil plugs, the base mold member is first set within a mold receptacle having inside dimensions registering with the dimensions of the base mold member. Next a soil and pre-polymer resin mixture is poured into the receptacle to cover the base mold member. It is possible to vibrate the receptacle to cause the high-viscosity mixture to spread out evenly. The tray mold member is then set within the receptacle and urged downward against the base mold member so as to force the soil-resin mixture into the plug-molding cavities. Clamped in this configuration, the mold members remain motionless with respect to one another while the soil-resin mixture swells and then cures to form the elastomeric soil plugs.
The carrying-tray method greatly facilitates automated handling of the soil plugs, from seeding to transplanting, while it eliminates "root system shock" experienced in other transplanting schemes. Root system shock is caused by broken root hairs, or exposure of roots to air for long periods of time.
The past method of manufacturing the soil plugs within the carrying trays, however, has its shortcomings. Sometimes the plugs do not completely fill the plug-molding cavities; the stunted plugs are then mishandled or mutilated by the automated handling equipment. At other times the plugs are formed with voids within the elastomeric medium, which introduces weak points into the plugs, hindering mechanical handling. Sometimes the plugs are loosened and slip from the carrying trays. And at all times too much of the expensive pre-polymer compound is wasted. But one shortcoming of the past manufacturing method stands out: it is too slow.